In recent years, light regulating glass that reversibly controls light by physicochemical means using compound glass incorporating functional materials has been proposed as an alternative to mechanical methods. Examples of such glass include liquid crystal, electrochromic, fine particle polarized orientation, photochromic and thermochromic types. In addition, heat absorbing glass and heat reflecting glass and so forth have come to be used in windows to prevent the entry of solar energy into living space. However, although heat absorbing glass and heat reflecting glass certainly prevent the entry of solar energy into living space, this glass leaves residual coloring and surface glare, thus having the disadvantage of losing the inherent advantages of glass in terms of being colorless and transparent. Moreover, in terms of energy conservation as well, control of visible light, which has roughly half of the energy of sunlight, is inadequate. Furthermore, as is described in detail in the research report of the 1991 Survey of Countermeasures of the New Glass Industry of the New Glass Forum (Measures for Preventing Global Warming), the use of light regulating glass is related to energy conservation policies, and there are strong expectations with respect to its future development.
The inventors of the present invention therefore focused their attention on solar energy irradiated onto windows. A comfortable living space could be obtained if window glass would cause a reversible change between transparency and opacity by automatically responding to the presence or absence of this energy. This self-responding quality is extremely appealing not only in terms of the characteristic of blocking out light from only the irradiated surface and its effect on energy conservation, but also with respect to installation, maintenance and upkeep costs. Although photochromic and thermochromic types could also be selected from this viewpoint, in contrast to the mechanism of action of photochromic types being complex and dependent on wavelength, thermochromic types are superior since they are dependent on thermal action only, thereby enabling temperature to be easily adjusted artificially as necessary. Furthermore, it should be noted that the wavelengths of solar energy that reaches the earth are within the range of 290-2140 nm, light within the range of visible light to near infrared light at a wavelength of 400-1100 nm accounts for roughly 80% of that energy, and the region of visible light is larger than that of near infrared light. This indicates that controlling sunlight in the region of visible light is important not only for screening the eyes, but also in terms of energy conservation and glare shielding effects. Furthermore, the present invention attempts to utilize the fact that, When an object is irradiated by light, that light is absorbed and converted into heat which causes the temperature of the object to rise due to that heat. In addition, the present invention may also be used by artificially controlling temperature with a heating element.
As indicated in the above-mentioned references and U.S. Pat. Nos. 4,132,464 and 4,169,661, the properties of the materials used in thermochromic types of light regulating glass are inadequate, and have yet to be put into practical use. It is therefore necessary to satisfy the following conditions for thermochromic glass to be used on a broad basis.
1. The phase change between transparency and opacity must be reversible. PA0 2. The reversible changes must be able to be repeated without phase separation. PA0 3. The temperature at which phase transition begins must be low. PA0 4. The glass must be colorless or the color of the glass must not change. PA0 5. The glass must be durable. PA0 6. The materials must not be toxic or cause environmental pollution.
The inventors of the present invention focused their attention on an aqueous solution that undergoes phase transition from a colorless, transparent state to a cloudy, opaque state caused by a rise in temperature of the aqueous solution as a self-responding material having the potential to satisfy these conditions. In addition, the present invention is also advantageous from the point of being fail-sale since it is transparent in the normal state, and becomes cloudy to block out light following the application of energy.
In the past, the cloud point phenomenon of non-ionic surface activators was widely known as an example of an aqueous solution that changes to a cloudy, opaque state due to temperature rise, and studies have been conducted using this as a thermochromic material. However, it goes without saying that this material is easily susceptible to phase separation, thus preventing it from satisfying conditions 1 and 2 described above. In addition, there are also certain isotropic aqueous solutions of non-ionic water-soluble polymers that become cloudy, and similar studies have also been conducted on these materials. However, these materials have also not been developed practically since these too are unable to satisfy the above-mentioned conditions 1 and 2. Although water-soluble polymer solutions are in the form of colorless, transparent aqueous solutions at room temperature, if left as is after having changed to a cloudy, opaque state following heating, phase separation and lack of uniformity of the aqueous solution occur. As a result, these solutions were unable to maintain their uniformity and permit repeated reversible changes. Moreover, when a laminated body was formed using this aqueous solution and allowed to stand vertically, the cloudy aggregate easily precipitated and separated due to the difference in specific gravity, thus preventing it from being used. Although it was particularly necessary to control the temperature at which clouding begins in order to enable practical use, this phenomenon became more remarkable when inorganic electrolyte was added. This is the result of the presence of ions causing the bonded water to be broken down and strengthening the hydrophobic bonds of water-soluble polymers resulting in greater bonding strength. In addition, when the concentration of water-soluble polymer was increased to effectively block out direct light, precipitation and separation also occurred easily due to aggregation. When the thickness of the low-concentration aqueous solution layer is increased to improve blocking of light, there are problems consisting of the occurrence of differences in convection current due to differences in temperature and the occurrence of contamination due to the splattering of low viscosity liquid during breakage.
However, the inventors of the present invention noticed that water-soluble polymers, and particularly aqueous solutions of polysaccharide derivatives having hydroxypropyl groups, Which exhibit a favorable balance of hydrophobic and hydrophilic properties, are able to adequately block out direct sunlight even when in the form of a thin film. The inventors of the present invention therefore focused their attention on a water-soluble polymer having a polysaccharide for its main chain and hydroxypropyl groups on its side chain. Hydroxypropylcellulose, which has structurally stable cellulose for its main chain, was initially selected as a representative example of such a compound, and detailed studies were conducted on its aqueous solutions.
As is already known, aqueous solutions having a concentration of 50 parts by weight or more of hydroxypropylcellulose demonstrate properties characteristic of cholesteric liquid crystal in the form of a lyotropic-type polymer cholesteric liquid crystal. This liquid crystal exhibits polar colored, spectral interference colors dependent on the angle of view caused by selective scattering of visible light. In addition, although the transition temperature shifts according to molecular weight, concentration, amount of electrolyte added and so forth, this liquid crystal demonstrates a reversible change, becoming opaque above a certain temperature. Additional studies were therefore conducted to determine whether this liquid crystal satisfied the four conditions mentioned above. The selective scattering wavelength of this liquid crystal demonstrates a red shift due to decreases in concentration of increases in temperature. Therefore, although contrivances were made such that the condition of colorlessness would be satisfied when the concentration was lowered to a concentration at which near infrared light was selectively scattered at roughly 20.degree. C. (for example, 56% by weight), selective scattering of red light was observed when viewed from an angle and when the temperature was roughly 10.degree. C., thus preventing the liquid crystal from satisfying this condition. Since this occurrence of changes in color depending on the viewing angle and temperature inhibits the degree of freedom of design in the design of buildings, automobiles and so forth, it is essentially impossible to use such a material practically. .Although the inventors of the present invention then attempted to further lower the concentration of the aqueous solution (for example, 52% by weight), the result was two different phases consisting of the liquid crystal phase and an isotropic phase. The liquid crystal demonstrated pale white scattering of light causing transparency to be remarkably impaired and preventing this liquid crystal from being used. Although these phenomena were due in part to molecular weight, they were observed in similar fashion in all cases. In addition, even when in the transparent state in which near infrared light is selectively scattered as viewed at a right angle, or in other words, from the front, there still was hazing caused by scattering and unevenness characteristic of the liquid crystal composition. Namely, a single domain state was not able to obtained over a large surface area, thus preventing the obtaining of transparency like that of glass. In addition, once the liquid crystal became frozen, unevenness of linear defects occurred in the cholesteric phase even after the liquid crystal was returned to room temperature.
In order to be able to use the liquid crystal in a wide range of applications such as buildings and automobiles over a large surface area in the manner of conventional glass, it is extremely important to be able to obtain the inherent transparency of glass without any dependency on angle of view. As a result of earnest studies on the part of the inventors of the present invention based on reexamination of isotropic aqueous solutions of polysaccharide derivatives that do not demonstrate dependency on angle of view while in the transparent or opaque states, it was found that reversible changes of isotropic aqueous solutions can be repeated without occurrence of phase separation, thus eliminating those defects that had remained as basic problems and heretofore prevented practical application.